Conventionally, various surface hardening methods are used to increase the surface hardness and to improve the abrasion resistance of iron and steel parts such as press dies and slide members and gears of machining tools.
The conventional surface hardening method for metal materials such as iron and steel includes pack carburizing, gas carburizing, liquid carburizing, high frequency quenching, flame hardening, plating, and nitriding.
Generally, the surface quenching is extensively used for hardening the surface of metal materials such as iron and steel. The conventional steel quenching is a modification (hardening) process in which a quenching object is heated to a temperature (800 to 1,300° C.) at which the inner solid solution body transforms to an austenitic structure with a face-centered cubic crystal structure and quenched to prevent transformation to ferrite, perlite, or bentonite, whereby a martensitic structure with a needle crystal structure consisting of fine plate or lenticular crystal is obtained in the austenitic structure. This process is named differently depending on the heating source.
Examples of the surface quenching include flame hardening, high frequency quenching, electron beam quenching, and laser quenching.
In flame hardening, the surface of an object to be quenched is heated to a specific temperature with acetylene and oxygen gas using a burner and then quenched. The flame hardening requires no special equipment but has a drawback that in the case of manual operation, the heating temperature cannot be controlled with accuracy and a skill is required to form a uniform hardened layer. Because of such high dependency on the operator's skill, flame hardening is considered to be beneficial for objects to be quenched having complex shapes such as gears and inefficient and inappropriate for objects to be quenched objects having simple shapes such as slide members of machining tools (see Patent Documents 1, 2, and 3 below).
In high frequency quenching, an object to be quenched is heated to a specific temperature using heat generated by a high frequency eddy current induced by electromagnetic induction and then quenched. This method utilizes such characteristic that the induced current is maximized on the surface of the object to be quenched and decreased toward the inner part thereof. Advantageously, the quenching property can efficiently be controlled by using a proper combination of frequency to induce an eddy current, material and shape of a heating coil, and cooling system according to the object to be quenched. However, this method has poor versatility (see Patent Documents 4, 5, and 6 below).
In electron beam quenching, an object to be quenched is heated to a specific temperature using an electric beam and then quenched. The quenching process is performed in vacuum which requires expensive equipment.
In laser quenching, an object to be quenched is heated to a specific temperature using a laser and then quenched. The laser quenching also requires expensive equipment like the electron beam quenching. Furthermore, it requires a troublesome task of applying absorbent such as graphite to the surface of an object to be quenched because a metallic object to be quenched reflects the laser.
In addition, Patent Documents 7 and 8 are referred to here, which relate to metal joint and disclose techniques to apply friction and stirring to metal as in the present invention although they are essentially different in purpose from the present invention relating to a metal surface hardening method and they utilize apparently different tools.    Patent Document 1: Japanese Laid-Open Patent Application Publication No. H05-230536;    Patent Document 2: Japanese Laid-Open Patent Application Publication No. H08-311636;    Patent Document 3: Japanese Laid-Open Patent Application Publication No. H11-131182;    Patent Document 4: Japanese Laid-Open Patent Application Publication No. 2002-372382;    Patent Document 5: Japanese Laid-Open Patent Application Publication No. 2005-2445;    Patent Document 6: Japanese Laid-Open Patent Application Publication No. 2005-307307;    Patent Document 7: International Application Publication WO93/10935; and    Patent Document 8: International Application Publication WO95/10935.